98 gmc yukon brakes help! I've done everything

[Mikeyslc]

Swapped a 14 bolt sf in the rear. I had spongy brakes before the swap but figured I'd deal with that after the swap was complete. I decided to put discs in the rear from lugnut4x4.com. bled the crap out of everything and still spongy. Call a brake shop who advises a master cylinder made for 4 wheel discs. So I bench bleed and replace the master with the correct gm part for 4 wheel discs. After bleeding for days this guy gives up and takes my money and gives me the "That's how these trucks brakes just are," speech. Well, my pedal goes right to the floor and I slowly roll to a stop. So on to the next guy who bleeds the abs and suggests maybe the module is bad, so we replace the module/pump. Still can't get a good pedal. So at this point I'm 4 calipers, 2 masters, new rubber lines, 18 bottles of fluid, new pump/module, and abs bled, with discs in the rear. I bought the tech 2 to try and move forward on my own because all these mechanics are running me dry on funds. But now, I literally don't know what my next move is. I'm trying to tow a boat and I can barely stop the truck by itself. Any suggestions?? I'm thinking pinch off all the rubber lines but one at a time and see if i can find a problem wheel? Maybe pressure bleed also?

 

[east302]

The stock system has a combination valve on the side of the ABS unit. It's what the brake lines connect to. This combination valve has a proportioning function which limits pressure to the front discs until the drum shoes have had time to react. Otherwise, your front discs would react before the drums.

The screwdriver is pointing to the combination valve below...

You must be registered for see images attach

If I were to guess, this proportioning function needs to be removed somehow so that you have equal pressure to all discs at the same time.

Try looking at some classic car forums where they've converted from drum to disc, focusing on the proportioning valve modifications. I'm sure someone has run across it before.

By the way, the ABS module (brake pressure modulating valve) does require an automated bleed with a scan tool to purge air from that component into the brake lines. It would then be followed by a regular bleed at all four corners, so at least the shop knew to do that.




Sent from my iPhone using Tapatalk

 

[leakers]

Did the second guy use a scan tool to bleed the ABS ? Or just try to manual bleed it ?

 

[TylerZ281500]

so ABS is a thing, theres bleeder screws on some of them depending on year that may not require a scanner to activate it.

wilwood adjustable prop inline could be a partial reason. what hoses were used on this? one bent or pinched? cheap hoses could be collapsing. how are you bleeding them?

spongy and stiff is one thing, pedal to the floor and wont stop is another, something isnt being done properly

 

[Gambles]

I did something like have some one hold that little springy button on the abs while having some one mash peddle and I bled the brakes, and it made a huge difference, but I also did drilled slotted rotors, fresh pads stainless brake lines, sanded drum and pads in the back, and the reverse slam on the brakes adjustment, so I can't say it was any one thing, only thing seems different is your rubber lines..

 

[east302]

Gambles brings up a good point. I had trouble getting fluid to the rear circuit on mine a while back. The combination valve had to be reset by holding a screwdriver against the pin on the side of the valve to keep it from extending while someone pumped the brakes. Don't push the pin in, just hold something flat against it.

It's under the black rubber cap...

You must be registered for see images attach

You must be registered for see images attach

Sent from my iPhone using Tapatalk

 

[Dubs]

I'm getting ready to do the rear disc conversion on mine. I have a 92. I went with SSBC. For mine you have to pull the piston out and remove the spring on the proportioning valve to create equal pressure front and rear. My truck has the ABS box but is not wired for ABS at the front calipers. I would jump online and do some more research. I'm sure there is a bleeder valve trick or a spring needs to be removed as stated above.

 

[Shinyrust]

Definitely need to press that little button under the rubber cap. Had the same thing. My uncle replaced a brake line and couldn't get it to bleed. Pressing the button will allow the fluid to go to the rear. It was a safety feature gm did.

Sent from my SM-G920T using Tapatalk

 

[professur]

Gambles brings up a good point. I had trouble getting fluid to the rear circuit on mine a while back. The combination valve had to be reset by holding a screwdriver against the pin on the side of the valve to keep it from extending while someone pumped the brakes. Don't push the pin in, just hold something flat against it.

It's under the black rubber cap...

You must be registered for see images attach

You must be registered for see images attach

Sent from my iPhone using Tapatalk

Do you only have the 'button' on one side? Reading what I've found online says there are two, but looking at mine (1998 C1500) I could only find one myself.

 

[east302]

My 98 K1500 only had one (on the fender side) but I've heard where some had one on the opposite end as well.

I'm not sure what the difference is.


Sent from my iPhone using Tapatalk